This code sets up a FastAPI application with specific configurations...

June 30, 2025 at 11:33 PM

processing_lock = asyncio.Lock()

app = FastAPI(
    title="BigQuery Table Viewer",
    description="Fetches tables from BigQuery and returns them as JSON",
)

# ─── CORS setup ────────────────────────────────────────────────────────────────
origins = ["http://localhost:3000"]
app.add_middleware(
    CORSMiddleware,
    allow_origins=origins,
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

# ─── Configuration from environment ────────────────────────────────────────────
PROJECT_ID = os.getenv("BQ_PROJECT_ID")
DATASET = os.getenv("BQ_DATASET")
if not PROJECT_ID or not DATASET:
    raise RuntimeError("Environment variables BQ_PROJECT_ID and BQ_DATASET must both be set.")

bq_client = bigquery.Client(project=PROJECT_ID)

@app.on_event("startup")
async def on_startup():
    await init_db()

async def get_db():
    async with SessionLocal() as session:
        yield session

This code sets up a FastAPI application with specific configurations and functionality. Here's a breakdown of what the code does:

processing_lock = asyncio.Lock():
- This initializes an asyncio lock to synchronize access to shared resources between coroutines. It can be used to prevent race conditions when asynchronously accessing or modifying shared data.
FastAPI application initialization:
- Creates a FastAPI application instance (app) with a title ("BigQuery Table Viewer") and description ("Fetches tables from BigQuery and returns them as JSON").
CORS setup:
- Configures Cross-Origin Resource Sharing (CORS) using the CORSMiddleware.
- It allows cross-origin requests from the specified origin: http://localhost:3000.
- Permits all HTTP methods (allow_methods=["*"]) and custom headers (allow_headers=["*"]), as well as credentials for secure communication.
Environment variable configuration:
- Fetches the BQ_PROJECT_ID and BQ_DATASET environment variables needed to connect to Google BigQuery. If either of these is missing, the application raises a RuntimeError.
BigQuery client setup:
- Initializes a BigQuery client (bq_client) from the Google Cloud SDK, configured to use the project specified in BQ_PROJECT_ID.
Startup event:
- Defines an asynchronous startup event handler (on_startup) that calls init_db(). This could be a function (not included in the provided code) that performs tasks like database initialization or setup before the application starts handling requests.
Database session management (get_db):
- Defines an asynchronous generator (get_db()) that yields a session from SessionLocal. This function is typically used as a dependency in API routes to provide database access.

Overall, this code sets up a basic FastAPI application for interacting with Google BigQuery, with proper initialization for middleware, environment configuration validation, database handling, and synchronization through the asyncio lock.

Generate your own explanations

Download our vscode extension

Read other generated explanations

Built by @thebuilderjr